Arrangement for supervising circuits

ABSTRACT

Apparatus for monitoring thermocouples comprises a Wien Bridge Oscillator with the thermocouple connected as a part of the bridge. The oscillator parameters are chosen so that oscillations do not occur during normal operation of the thermocouple. The frequency of oscillations are determined by an RC-network. An indicating device is controlled by the output voltage of the oscillator.

United States Patent 1191 UNITED STATES PATENTS lmlet 323/75 H HohlerAug. 28, 1973 ARRANGEMENT FOR SUPERVISING 2,673,960 3/1954 Doblmaier323/75 K CIRCUITS 3,614,598 l0/l97l Meyer 323/75 H [75] Inventor: AlbertHohler, Kassel, Germany FOREIGN PATENTS QR APPLICATIONS [73] Assignee:U.S. Philips Corporation, New York, 1,059,107 11/1959 Germany 307/117[22] Filed: Dec. 3, 1971 Primary Examiner-Herman .l. Hohauser AssistantExaminer-Morris Ginsburg [21] Attorney-Frank R. Trifari et al.

[30] Foreign Application Priority Data Dec. 19, 1970 Germany P 20 62788.7 [57] ABSTRACT Apparatus for monitoring thermocouples comprises a[52] U.S. Cl 307/117, 323/75 H, wien Bridge Oscillator with thethermocouple Com [5 1] Int Cl Gosh 21/00 nected as a part of the bridge.The oscillator parame- [58] i 75 H 75 ters are chosen so thatoscillations do not occur during I 323/785 H i normal operation of thethermocouple. The frequency of oscillations are determined by anRC-network. An indicating device is controlled by the output voltage of[56] References Cited the oscillator 7 Claims, 1 Drawing FigurePATENTEI] "I628 I975 INVENTOR.

ALBERT HOHLER AGENT ARRANGEMENT FOR SUPERVISING CIRCUITS This inventionrelates to an arrangement for monitoring circuits, in particular fortemperature-control devices and measuring converters using thermocouplesensors. An arrangement of this type generally comprises an oscillatorwhich in normal operation does not oscillate and an indicating devicewhich is controlled by an output voltage supplied by the oscillator inthe case of a fault.

Such a circuit arrangement is described, for example, in German Pat. No.1,059,107. In this arrangement the oscillator coil is provided with anadditional short circuit winding which, through a capacitance, isconnected in parallel with the line part to be supervised. When themeasuring circuit is closed, the oscillator is damped so heavily that itdoes not oscillate. When the circuit, for example, the thermocouplecircuit, is broken, oscillations set in which, after rectification,produce a signal in the indicating circuit.

This arrangement, however, suffers from the disadvantage that the linecapacitance may have widely different values which, depending on thetype and the length of the cable, may range from less than 1 nanofaradto about 50 nonofarad. Undesirable resonance effects of .theshort-circuit winding with the line capacitance or resonances in theline itself may prevent the oscillator from starting in spite of therupture of a thermocouple.

It is an object of the present invention to preclude such operationalfaults. This is achieved in that the oscillator is connected as anRC-generator and in that a capacitor-resistor network in the form of aWien bridge the parallel branch of which is constituted by the measuringcircuit is provided as the element which determines the frequency andthe initiation of oscillation.

An embodiment of the invention will now be described, by way of example,with reference to the accompanying diagrammatic drawing, the singleFIGURE of which shows a Wien Bridge Oscillator arrangement forsupervising a thermocouple circuit.

Referring now to the FIGURE, a thermocouple 1 with an internalresistance 2 is connected through a line having an internal resistance 3and a line capacitance 4 to the input terminals of an amplifier 5 of theWien Bridge Oscillator. Together with an appropriately chosencapacitance 6 connected in parallel with the input terminals of theamplifier 5, this circuit forms the parallel branch of a Wien bridge.The input terminal of the amplifier 5 which is indicated by a plus signis connected to the output terminal of the amplifier 5 through theseries connection of a resistor 8 and a capacitor 7. The plus sign atone of the input terminals of the amplifier indicates that the outputvoltage and the input voltage of the amplifier are in phase. If theseries combination of the resistor 8 and the capacitor 7 is termed acomplex resistance Z and the parallel connection of the resistors 2 and3 with the capacitors 4 and 6 is termed a complex resistance 2,, wehave:

oscillation.

For the initiation of oscillation the self-excitation for-.

mula holds:

, k V l where V is the amplification factor of the amplifier. If theimpedance Z, is given a value from 1 to 2 k9, with an operatingresistance of the thermocouple circuit of from 10 to 20 ohms and anamplification factor of less than 10, undesirable starting of theoscillator and hence a false alarm may be prevented with a high degreeof certainty. From the above the value of the permissible linecapacitance and hence of the available length of the line may bededuced. With a capacitance of 50 nanofarad in both branches and whenthe resistance of the thermocouple circuit is raised to 1 k0 (resistance8 1 kn) the frequency produced would be about 16 kHz according to theformula f 1/2 a- R -C. Thus. the amplification factor required for theamplifier would be at least 3.

In the case of a cable capacitance of pF/m and a parallel capacitance of20 nF the permissible line length would be 250 m. I

The said frequency of about 16 kHz is particularly advantageous since inthis frequency range the voltage distribution over the line may still beconsidered to be stationary, i.e., the cable capacitance may be regardedas a lumped capacitance.

Designing the complex voltage divider so that it has an overallimpedance of about from 1 to 2 k0 is advantageous since it enables theavailable power of the amplifier to be maintained small.

Capacitive bias loading of the thermocouple circuit by the linecapacitance 4 is appropriate to ensure that even in the case of a smallcable length oscillation is ini tiated only at an appreciable increasein resistance.

Furthermore, it is advantageous to set the amplification factor of theamplifier 5 at a fixed value and to stabilise it against fluctuations ofthe temperature and the supply voltage by the provision of negativefeedback.

The measuring converter, recording instrument and- /or regulator 11connected to the sensor line must have an input impedance for thefrequency produced in the case of faults such as to enable a resistanceincrease in the thermocouple line to take effect.

In the case of rupture of the thermocouple the highfrequency alternatingvoltage produced is rectified and is available as a signal voltage of afew volts (direct voltage) at terminals 9 and 10. In the arrangementshown this signal voltage is applied through a large resistor to theinput of the measuring converter, record ing instrument and/or regulator11 connected to the thermocouple circuit. This ensures that, dependingon the polarity of the rectifier, the measuring device 11 is set eitherto the positive or to the negative final position so that, in the caseof an indicator or a recording instrument, a visible signal is given andin the case of a control device the line controlled is switched out ofthe circuit.

. I claim:

1. Apparatus for monitoring an electric element comprising, anamplifier, a plurality of resistors and capacitors, means connectingsaid resistors and capacitors in an RC network and in circuit with saidelement and said amplifier to form a Wien Bridge Oscillator which isarranged so as not to oscillate during normal operation of the electricelement, said RC network determining the frequency of oscillation andthe start of oscillation being determined by a change in the resistancevalue of said electric element when a defect occurs therein, and anindicating device controlled by the output voltage of the oscillator soas to indicate the occurrence of a defect in said electric element.

2. Apparatus as claimed in claim 1 wherein the resistors and capacitorsof said RC network are chosen as to produce a frequency of oscillationof about 16 KHz upon the occurrence of a fault in said electric element.

3. Apparatus as claimed in claim 1 further comprising circuit means forproviding an adjustable negative feedback for said amplifier.

4. Apparatus as claimed in claim 1 wherein said indicating devicecomprises a recording instrument connected in parallel with the inputterminals of the amplifier and having a high input impedance.

5. Apparatus as claimed in claim 1 wherein said RC network includes afirst branch of the bridge having a capacitor and resistance meansconnected in parallel to the input terminals of the amplifier, andwherein the resistance means includes said electric element.

6. Apparatus as claimed in claim 5 wherein said RC network furthercomprises a second branch of the bridge that includes a resistor and asecond capacitor plifier and an input terminal of the indicating device.

1. Apparatus for monitoring an electric element comprising, anamplifier, a plurality of resistors and capacitors, means connectingsaid resistors and capacitors in an RC network and in circuit with saidelement and said amplifier to form a Wien Bridge Oscillator which isarranged so as not to oscillate during normal operation of the electricelement, said RC network determining the frequency of oscillation andthe start of oscillation being determined by a change in the resistancevalue of said electric element when a defect occurs therein, and anindicating device controlled by the output voltage of the oscIllator soas to indicate the occurrence of a defect in said electric element. 2.Apparatus as claimed in claim 1 wherein the resistors and capacitors ofsaid RC network are chosen as to produce a frequency of oscillation ofabout 16 KHz upon the occurrence of a fault in said electric element. 3.Apparatus as claimed in claim 1 further comprising circuit means forproviding an adjustable negative feedback for said amplifier. 4.Apparatus as claimed in claim 1 wherein said indicating device comprisesa recording instrument connected in parallel with the input terminals ofthe amplifier and having a high input impedance.
 5. Apparatus as claimedin claim 1 wherein said RC network includes a first branch of the bridgehaving a capacitor and resistance means connected in parallel to theinput terminals of the amplifier, and wherein the resistance meansincludes said electric element.
 6. Apparatus as claimed in claim 5wherein said RC network further comprises a second branch of the bridgethat includes a resistor and a second capacitor connected in seriesbetween an output terminal and an input terminal of the amplifier. 7.Apparatus as claimed in claim 6 wherein the input terminals of theindicating device are connected to the input terminals of the amplifier,and means providing a feedback path between an output terminal of theamplifier and an input terminal of the indicating device.